4mo ago

PCIe bifurcation

Hey y'all, I bought a x4x4x4x4 4 slots M.2 PCIe card foolishly thinking that if it fits in the slot, it would surely work. In the end, I got 2 of the 4 SSDs working on my old AM4 X470 chipset. I'm coming to you for advice what the cheapest way to get to use this would be. I've noticed that a lot of CPUs have a PCIe lane limitation of 28, just short of what I need (I'd like to run the SSDs but also a x16 GPU). I'm not too keen to buy a threadripper setup for this occasion...

Cheers!

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I'm not sure what hardware you're running, but with my motherboard, to get 4x4x4x4 out of a slot requires sacrificing GPU bandwidth from x16 to x8
to get 4x NVMe drives out of a single PCIe slot without bifurcation you need a card that has it's own RAID controller. These aren't cheap (think ~$500) as they are specialty hardware, but it's a hell of a lot cheaper than a whole professional workstation or server.
- You don't necessarily need a raid controller, I think a pcie bridge or switch chip will work too. They're still expensive but they're significantly less than $500.
- This sounds like a path I could take - just buying a mainboard like that and making it a NAS. Which board do you have there? If it's AM4, I could get a similar one and just reuse my old CPU.

i don't think you have much options. cheapest = use gpu in x8. you could try an adapter to plug the ssd into sata port, but no idea if that would even work. just for the fun of it, i looked at my 7800x3d specs and under connectivity it states that 24 pcie lanes are usable of a total of 28, but mobo provides an additional 8 or 16 more depending on the chipset. wondering if that results in 32 or is it limited to 28...

It’s all down to your motherboard. But the tldr is no, at least not without compromises.
The top x16 slot is almost always connected directly to the CPU. The others are limited to X8 or X4.
Your CPU has 20 lanes. 16 for the GPU, and 4 for your “main” NVME SSD. Your chipset typically has like 4 or 8 more (the very bottom slot). And you have to figure out some sort of combo where that works, and they’re not combineable. What’s worse is a lot of motherboards hard wire those extra x4 pcie lanes to the M.2 SSD slots so you get no choice.
- ur “main” NVME SSD. Your chipset typically has like 4 or 8 more (the very bottom slot). And you have to figure out some sort of combo where that works, and they’re not combineable. What’s worse is a lot of motherboards hard wire those extra x4 pcie lanes to the M.2 SSD slots so you get no choice.
  I've put the bifurcation card next to the CPU now and it only offers two x4 lanes. I thought zen2 should be able to handle bifurcation, so if it was directly hooked up with the CPU, I should get all four. Seems chipsets still have some influence over the first PCIe slot.
  
  What mobo do you have? That's the most important thing here. Your mobo will almost always be the limiting factor in bifrucation.
  When I was looking around before I saw that Asus supports 4 way bifrucation. I got bored so I asked chatGPT and it thinks only two x470 mobos support 4 way bifrucation. ASUS ROG Crosshair VII Hero (Wi-Fi) and MSI X470 Gaming M7 AC. It also seems to think x570 has better support, but I think that's more just that they support two way vs no bifrucation at all.
  4 way bifrucation isn't a very common thing for a regular user to do which is probably why so few boards support it. 4 way plus a full fat GPU is even less common. To be honest I'm kinda surprised any 400 series boards support bifrucation at all.

What’s the motherboard model? Which slot are you attempting to use? Is it a physical X16 slot with only a X8 connection? Do you have any other slots available for M.2 drives?
- It's a MSI X470 GAMING PRO. Tried both upper x16 slots. The second one seems to be x8, but the first one doesn't work either despite being x16. I have other m.2 slots but they're already used up - however those are smaller SSDs so worst case I could move my OS to a SATA drive and populate those m.2 slots with the larger and newer SSDs that i bought! 😂

if it fits in the slot, it would surely work
If you got something like the ASUS Hyper M.2 x16 Gen 4 Card:
https://www.amazon.com/ASUS-M-2-X16-Expansion-Card/dp/B084HMHGSP
https://www.asus.com/us/motherboards-components/motherboards/accessories/hyper-m-2-x16-gen-4-card/
Then there are specific motherboards listed that support the bifurcation requirements.
Of course, if your x16 slot is taken up with this thing, you can no longer use the slot for a graphics card.
Perhaps the cheapest way forward would be to use your x16 slot for graphics, and distribute your 4 M.2 cards using multiple adapters in the smaller slots (if you have room for that). One of the adapters might need to be PCIe x1. Well, cheaper than Threadripper!
- That's precisely the SSD PCIe solution that I bought. ^^ Yeah, might have to redistribute... Thanks for letting me know about the compatibility listing, I wasn't aware of it. I'll check if I can get one of those second hand.
  Edit: I'm slightly worried about ASUS as a mainboard supplier though because of their recent rootkit escapades...

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